The present invention relates to an imaging apparatus having an optical system for forming an image of an object, of the type having a focusing device provided with detector means for determining deviations between the actual image surface, which has a predetermined position with respect to the optical system, and a second surface on which the image of the object is to be formed. Such apparatuses are known in which the focusing device has means fast with the optical system and constructed to focus a spot of monochromatic light under a large angle of incidence onto said actual image surface and means for collecting the light reflected by said second surface and focusing it on a differential light detector which is so located that the image of the spot on the detector is centered on the detector when the two surfaces coincide.
The invention is particularly suitable for use in equipment used for manufacturing integrated circuits, and notably in the equipment known as "step and repeat device" or wafer stepper, designed to form repetitively the image of a reticle on successive areas of the surface of a semiconductor wafer, then, the surface of the wafer constitutes the second surface.
The distance between the lens system of the photorepeater, and the surface should be accurately adjusted for fear of degradation of the image. By way of example, the distance between the lens and the surface is 6 mm in a current commercial step and repeat apparatus and the tolerance is then 2 microns to project a mask pattern having 1 micron wide lines.
Imaging apparatuses suitable for use as photorepeater have been used which include focusing devices operating automatically. Such devices exist in wafer steppers manufactured by G.C.A. since 1977 at least. The construction and operation of the detector means are illustrated in FIGS. 1 and 2. A monochromatic light source 10 is associated with lens 11 which forms an image 12 of the source which is on the surface of the wafer 13 when the latter is in its set position, as indicated in FIG. 1. Light collecting means, shown diagrammatically in the form of a lens 14, form an image of the light spot on the surface 13 which, when the wafer 13 is in its nominal position, is centered with respect to a differential optoelectronic detector 15; that detector may for instance have a two-quadrant diode, or two separate diodes. A differential error or deviation signal is obtained which is fed to a servo system.
Referring to FIG. 2, any deviation in the altitude .DELTA.z of the wafer 13 causes an offset .DELTA.x of the image in the differential photodetector 15, hence an error signal whose value is an increasing function of .DELTA.x and whose sign depends on the direction of .DELTA.z.
Such an autofocus device can be very sensitive, particularly if the light beam has a large angle of incidence. However, it has a serious drawback. Any irregularity in reflection by the surface of the wafer 13 results in an asymmetry in the brightness distribution of the spot and can result in a focusing error.
Attempts have been made to overcome the difficulty by making the light follow the same path in one direction, and then the opposite direction by replacing detector 15 with a mirror reflecting it to a detector positioned close to the source 10 (Solid State Technology, June 1980). This solution is not however entirely satisfactory due to associated construction problems.
It is an object of the invention to provide an improved imaging apparatus of the above-defined type. It is a more particular object to provide an imaging apparatus having an automatic focusing device overcoming the problems associated with reflection defects of the surface on which the image should be formed and however simple.
An apparatus according to the invention has two sets of said detector means, corresponding to reverse paths of the light and means are provided for summing the differential signals supplied by the detectors of the two sets.
For the two paths to coincide, it will typically be necessary to provide beam splitters, such as semitransparent strips, for reflecting light from the light source of one of the sets while transmitting light to the detector of the other set.
The device preferably further comprises detector means constructed and associated for sensing angular deviations, that is to say attitude deviations, between the second surface and an ideal angular position. The additional means may include a source to direct an oblique parallel beam of light onto the surface and means for focusing the image of the source onto a four quadrant photodetector; processing of the output signals from the four quadrants makes it possible to deliver attitude error signals indicating angular deviations about two mutually perpendicular axes.
It is another object of the invention to overcome a difficulty which is specific to wafer steppers. When a reticle or mask is handled to print images thereof in several successive zones on a semiconductor wafer, care should be taken to avoid depositing particles on the reticle whose image would be reproduced in the integrated circuit. At the present time, equipment for reticle inspection by photometric comparison of two homologous areas of adjacent patterns is known. However, that approach has drawbacks. It is in fact the reticle that is checked, rather than the image thereof in the plane of the wafer. Moreover, there remains a risk that dust is deposited during the transfer from the checking apparatus to the wafer stepper.
For overcoming the difficulty, the wafer stepper has means for forming two homologous areas of adjacent patterns on two photoelectric detectors and for measuring the difference between the output signals from the two detectors.
The approach can be used each time several elementary patterns of integrated circuit exist on the same reticle, which is frequently the case since the time duration of the step and repeat operation on the wafer is thus reduced. The actual image and not the reticle is examined directly, whereby defects printed on the silicon wafer are taken into account as well and reticles are found acceptable which would be rejected in a conventional inspection apparatus. The defect detection resolution is of the same order of magnitude as the resolution of the lens, that is equal or even less than one micron.
The various components of the device may be varied in position with respect to those which serve directly for imaging. For example, the angular deviation detection means (levelling means) may use a light path in a plane forming an angle, for example a right angle, with the plane of the light paths in the sets of means for autofocusing.
The invention will be better understood from the following description of particular embodiments of the invention, given by way of examples.